Deprecate android_is_in_vendor_process

android_is_in_vnedor_process previously checked if current process is in
system(-ish) section by trying to open vndk namespace. However, this
would not work once VNDK is deprecated. As there is no clear way to
replace this, and there is few usage on this API, it would be better
deprecated, and be implemented from each module if required.

Bug: 300366609
Test: AOSP CF build and boot succeeded
Change-Id: I70c70efb36199e12d4877ea4ecb39dbf94653551